Supercapacitors, alternatively known as ultracapacitors, electrical double layer capacitors or electrochemical capacitors, are energy storage devices that have considerably more specific capacitance than conventional capacitors. Low resistance supercapacitors are ideally suited for high power applications for mobile devices, particularly those using GSM (Global System for Mobile communication) and GPRS (General Packet Radio Service) wireless technologies.
Supercapacitors can play a role in hundreds of applications. The energy and power storage markets, where supercapacitors reside, are currently dominated by batteries and capacitors. It is well recognised that batteries are good at storing energy but compromise design to enable high power delivery of energy. It is also well recognised that capacitors enable fist (high power) delivery of energy, but that the amount of energy delivered is very low (low capacitance). Overlaying these limitations of existing batteries and capacitors against market demand reveals the three main areas of opportunity for supercapacitors, battery replacement, devices which have higher energy density, bad complements, devices which have high power and energy densities; and capacitor replacement, devices which are smaller and not only have high power density but have high frequency response.
Currently, the relatively high power density of supercapacitors make them ideal for parallel combination with batteries that have high energy density to ram a hybrid energy storage system. When a load requires energy that is not constant, complementing the battery with a supercapacitor allows the peaks to be drawn from the charged-up supercapacitor. This reduces tie load on the battery and in many cases extends the lifecycle of a battery as well as the lifetime of rechargeable batteries.
Modern mobile devices require power systems that arm capable of dealing with large fluctuations in the load. For example, a mobile telephone has a variety of modes each with a different load requirement. There is a stand-by mode, which requites low power and is relatively constant. However, this mode is periodically punctuated by the need to find the nearest base station and a signal is sent and received, requiring a higher load. In full talk mode where continuous contact to a base station is required, the load takes the form of a periodic signal where the instantaneous load is quite different from the average. A number of communication protocols exist, such as GSM and GPRS, but they are all characterized with a periodic load. The parallel supercapacitor-batty hybrid is particularly suited to this application because the power from the supercapacitor is used during the high loads that are usually short in duration and the energy from the battery can recharge the supercapacitor and supply a base load during the time of low power demand. As further miniaturization of digital wireless communication devices occur, leading to decreased battery sizes, the need for supercapacitors will increase.
Supercapacitors also have application in the field of Hybrid Electric Vehicles (HEV). Supercapacitors can be used as an integral component of the drivetrains of these vehicles and are used as the primary power source during acceleration and for storage of energy reclaimed during regenerative braking. Such vehicles could conceivably halve a motorist's fuel bill and slash emissions by up to 90%.
Capacitance arises when two parallel plates are connected to an external circuit and a voltage difference is imposed between the two plates, the surfaces become oppositely charged. The fundamental relationship for this separation of charges is described by the following equation
  C  =            ɛ      ⁢                          ⁢      A        L  where C denotes capacitance with a unit of farads (F), ε is the permittivity with a unit of farads per metre (m), A is the area of overlap of the charged plates and L is the separation distance. The permittivity of the region between the plates is related to the dielectric constant of the material that can be used to separate the charged surfaces.
The problem with exiting commercial capacitors using conventional materials is that their performance is limited by their dimensions. For example, a capacitor based around a metallized coating of a polyethylene sheet that is 50 μm thick will develop only 0.425 μF for one square metre of capacitor. Thus, over 2.3 million square metres will be required to develop 1 F.
The supercapacitors developed by the present applicant are disclosed in detail in the applicants copending applications, for example, PCT/AU98/00406, PCT/AU99/00278, PCT/AU99/00780, PCT/AU99/01081, PCT/AU00/00836 and PCT/AU01/00553, the contents of which are incorporated herein by reference.
These supercapacitors developed by the applicant overcome the dimensionality problem described above by using as a coating material an extremely high surface area carbon.
These supercapacitors include two opposed metal electrodes. These electrodes are coated and are maintained in a predetermined spaced apart electrically isolated configuration by an intermediate electronically insulating separator. In very broad terms, the electrodes form current collectors for the coating material, in that the metal offers significantly less resistance than the coating material. The coatings typically formed from a particulate carbon or carbons and a binder used for adhering the carbon to itself and to the associated current collector.
The coated electrodes and intermediate separator can be either stacked or wound together and disposed within a housing that contains an electrolyte. Two current collecting terminals are then connected to and extend from respective electrodes for allowing external access to those electrodes. The housing is sealed to prevent the ingress of contaminants and the egress of the electrolyte. This allows advantage to be take of the electrical double layer that forms at the interface between the electrodes and the electrolyte. That is, there are two interfaces, those being formed between the respective electrodes and the electrolyte. This type of energy storage device is known as a supercapacitor. Alternatively, these have been known as ultracapacitors, electrical double layer capacitors and electrochemical capacitors.
The electrolyte contains ions that are able to freely move throughout a matrix, such as a liquid or a polymer, and respond to the charge developed on the electrode surface. The double layer capacitance results from the combination of the capacitance due to the compact layer (the layer of solvated ions densely packed at the surface of the electrode) and the capacitance due to the diffuse layer (the less densely packed ions further from the electrode).
In supercapacitors, the compact layer is generally very thin, less than a nanometre, and of very high surface area. This is where the technological advantage for supercapacitors over conventional capacitors lies, as charge storage in the extremely thin compact layer gives rise to specific capacitances of approximately 0.1 Fm−2. This is an increase by several hundred thousand-fold over conventional film capacitors. As well, the applied potential controlled, reversible nanoscale ion adsorption/desorption processes result in a rapid charging/discharging capability for supercapacitors.
The electrode material may be constructed as a bed of highly porous carbon particles with a very high surface area. For example, surface areas may range from 100 m2 per gram up to greater than 2500 m2 per gram in certain preferred embodiments. The colloidal carbon matrix is held together by a binding material that not only holds the carbon together (cohesion) but it also has an important role in holding the carbon layer onto the surface of the current collecting substrate (adhesion).
The current collecting substrate is generally a metal foil. The space between the carbon surfaces contains an electrolyte (frequently solvent with dissolved salt). The electrolyte is a source of ions which is required to form the double layer on the surface of the carbon as well as allowing ionic conductance between opposing electrodes. A porous separator is employed to physically isolate the carbon electrodes and prevent electrical shorting of the electrodes.
The energy storage capacity for a supercapacitor can be described by the equation
  E  =            1      2        ⁢          CV      2      where E is the energy in joules and V is the rated or operating voltage of the supercapacitor. Apart from the voltage limitation, it is the size of the supercapacitor that controls the amount of energy stored, and the distinguishing feature of supercapacitors are the particularly high values of capacitance. Another measure of supercapacitor performance is the ability to store and release the energy rapidly; this is the power, P, of a supercapacitor and is given by
  P  =            V      2              4      ⁢      R      where R is the internal resistance of the supercapacitor. For capacitors, it is more common to refer to the internal resistance as the equivalent series resistance or ESR. As can be deduced from the foregoing equations, the power performance is controlled by the ESR of the entire device, and this is the sum of the resistance of all the materials, for instance, substrate, carbon, binder, separator, electrolyte and the contact resistances as well as between the external contacts.
One variable of interest in the field of supercapacitors that has yet to be fully explored is the nature of the electrolyte involved. The electrolyte is typically one or more solvents containing one or more dissolved ionic species. In many cases, the physical and electrochemical properties of electrolyte are a key factor in determining the internal resistance (ESR) of the supercapacitor and the “power spectrum” of the supercapacitor, ie the ability of the supercapacitor to provide power over various time domains or in various frequency ranges.
The factors influencing the conductance (κ) of an electrolyte solution are described in detail in an article by B. E. Conway taken from “The Fourth International Seminar on Double Layer Capacitors and Similar Energy Storage Devices”, Dec. 12-14, 1994, held at Ocean Resort Hotel and Conference Centre, Deerfield Beach, Fla. and co-ordinated by Florida Educational Seminars, Inc., 1900 Glades Road, Suite 358, Boca Raton, Fla. 33431.
In summary, there are two principle factors which are involved in determining the conductance—these are:    a) the concentration of free charge carriers, cations and anions; and    b) the ionic mobility or conductance contribution per dissociated ion in the electrolyte.
There are a number of sub factors which in turn influence these two principle factors. These are:    The solubility of the selected salt.    The degree of dissociation into free ions and factors such as the extent of ion-pairing of the ionic species. This in turn is influenced by the salt concentration, temperature and the dielectric constant of the solvent.    The viscosity of the solvent, which is a temperature dependent property. As temperature increases, there is a corresponding decrease in viscosity.
Solvents for supercapacitors can thus be designed with the following criteria in mind:    Solvent for selected ionic species    Degree of dissociation of cation/anion pairing in solution    Dielectric constant    Electron-pair donicity    Permits high ion mobility    Extent of solvation of free ions and radii of solvated ions    Temperature coefficient of viscosity (ie low viscosity in the intended temperature range) and ion pairing equilibria.
There is also the necessity for the solvent to be chemically stable. Aqueous based electrolytes, such as sulfuric acid and potassium hydroxide solutions, are often used as they enable production of an electrolyte with high conductivity. However, water is susceptible to electrolysis to hydrogen and oxygen on charge and as such has a relatively small electrochemical window of operation outside of which the applied voltage will degrade the solvent. In order to maintain electrochemical stability in applications requiring a voltage in excess of 1.5V, it is necessary to employ supercapacitor cells in series, which leads to an increase in size in relation to non-aqueous devices. Stability is important when one considers that the supercapacitors must charge and discharge many hundreds of thousands of times during the operational lifetime of the supercapacitor.
There are of course processing requirements on the solvent also, such as cost, toxicity, purity and dryness considerations.
Non aqueous solvents commonly used in related fields, eg batteries, can be classified as: high dielectric constant aprotic (e.g. organic carbonates), low dielectric constant with high donor number (e.g. dimethoxyethane, tetrahydrofuran or dioxolane), low dielectric constant with high polarisability (e.g. toluene or mesitylene) or intermediate dielectric constant aprotic (e.g. dimethylformamide, butyrolactone) solvents.
However, in addition to the specific electrolyte requirements of supercapacitors mentioned above, there is also the practical consideration that supercapacitors do not operate in isolation. Rather, in use, they are in confined environments in the presence of components which generate high temperatures, and like the other components, this must be borne in mind when selecting the electrolyte solvent. Also, it needs to be borne in mind that the supercapacitors must be capable of operation at start-up at temperatures much lower (even into the sub zero range) than the high operating temperatures referred to above.
The energy storage of batteries, in contrast to the power delivery of supercapacitors, is not critically dependent on the contribution of the electrolyte to the ESR of the cell, although even in batteries, low ESR is desirable. Solvents which have high boiling points invariably have high viscosities, and consequently, low charge mobilities at low temperatures. High boiling solvents, such as cyclic ethers and lactones can therefore be used in batteries with less regard to what would be an unacceptably high ESR in supercapacitors.
FIG. 1 shows the relationship between literature boiling point and viscosity for a number of substances.
FIG. 2 shows the relationship between conductivity and reciprocal solvent viscosity at 25° C. for 0.65M tetraethylammonium tetrafluoroborate (TEATFB) for a variety of solvents. Source: Makoto Ue, Kazuhiko Ida and Shoichiro Mori; “Electrochemical Properties of Organic Liquid Electrolytes Based on Quaternary Onium Salts for Electrical Double-Layer Capacitors.” J. Electrochem. Soc., Vol. 141, No. 11, November 1994
FIG. 3 is a plot of ESR and reciprocal conductivity, where the conductivity is varied by changing the concentration of TEATFB in acetonitrile, and shows in a general way the relationship between ESR and conductivity for a supercapacitive cell.
These three Figures also serve to illustrate the other relationships that exist between the properties, such as boiling point and ESR, viscosity and ESR and boiling point and conductivity.
Admixing a low boiling fluid and a high boiling fluid may appear to be an attractive option, with the low boiling, low viscosity compound providing acceptable charge mobility at the low end of the temperature range, and the high boiling component reducing in viscosity and providing charge mobility at higher temperatures. In practice, however, this approach is generally not viable because while acceptable results may be achieved at ambient temperatures, at higher temperatures the low boiling component will fractionate out. Fractionation can present a challenge to the mechanical integrity of the supercapacitor packaging.
It is an object of the present invention to provide a non-aqueous solvent suitable for use in the energy storage device which overcomes one or more of the abovementioned disadvantages, or at least provides a commercially viable alternative.